A prerequisite of micro-circuit production with a reasonable yield is defect free masks and wafers to be used in the production process. Over the past 12 years a number of optical systems have been developed, and patented for the automatic inspection of optical masks and wafers. (See U.S. Pat. No. 4,247,203, 4,805,123, 4,618,938 and 4,845,558). These systems optically perform a comparison between two adjacent dice on a photomask, reticle or wafer. Similarly, technology has evolved to inspect a die against, a CAD database (See U.S. Pat. No. 4,926,487). These optical systems are, however, limited to optical masks because defects on X-Ray masks may not be apparent in the visible or ultraviolet spectrum. Furthermore, optical inspection is limited in its resolution capability by the fundamental diffraction limit that of course also limits optical lithography. Even with phase shift mask techniques it is expected that line widths below 0.35 microns cannot be achieved with optical lithography techniques and that X-Ray lithography will dominate for line widths smaller than that.
Advances in optical lithography for the manufacturer of microcircuits have permitted increasingly smaller and smaller line widths. As an example, for a 256 megabyte DRAM, the line width on the wafer, is between 0.25 and 0.35 micrometers. For line widths of this size, phase shift masks are used in the manufacture of the semiconductor device. These masks typically have a quartz base, a patterned chromium layer on the surface thereof, and phase shift wells selectively etched into the substrate. A phase shift may also be produced by patterned, optically transparent, material placed on the quartz substrate or the chromium layer of the phase shift mask.
The inspection of masks that are used to produce very fine line widths on the semiconductor to be produced, such as the type of masks referred to above, requires the detection of defects in the patterned chromium layer, as well as the measurement of the depth of trenches or wells, both wanted and unwanted, in the quartz. Additionally, it is necessary to be able to detect the presence or absence of defects in the quartz which is also referred to in the art as the phase shift material.
Typically, phase shift masks, because they are optically transmissive and designed for use optically, are inspected using optical techniques. Those optical inspection techniques have proven to be frequently inadequate for the masks that can produce newer fine-line patterns because of the limitations in resolution of the prior art optical inspection methods.
The co-pending application from which this application is a continuation-in-part application, as identified above, describes an electron microscope inspection system and the use of that system to inspect primarily x-ray masks and wafers. One shortcoming of electron scanning in those systems in the past is that they did not measure the optical phase shift related to the depth of a well or trench in a known material of the mask or wafer.